It is known to provide a sleeve plug assembly to be attached to a spindle head on a metal working machine, the plug portion of the assembly being adapted to receive a tool. A known device of this general type includes a locking bushing attached to a plug, and a holding screw which must be screwed radially into the spindle head, the head of the screw also penetrating the locking bushing. As will be evident, if a suitable internally threaded hole of the proper size is not already present in the spindle head at the proper location, it would be necessary to provide one. Also, it will be seen that sufficient space laterally of the spindle head must be present to permit attachment of the holding screw into the spindle head and locking bushing.